[GitHub] [openwhisk] style95 commented on pull request #5221: Change the value of pause-grace for new scheduler

[GitBox <gi...@apache.org>]

style95 commented on PR #5221:
URL: https://github.com/apache/openwhisk/pull/5221#issuecomment-1106074614

   > 
   
   Let me share the details.
   In FPCScheduling, each ContainerProxy keeps sending `FetchRequest` to schedulers to pull activations.
   When there is no activation for `pause-grace`, the container would be paused in turn it can't send the `FetchRequest`.
   Warmed containers are still counted in the scheduler but they cannot send the request accordingly no activation is pulled.
   After then, once a new activation comes to an action queue, it would try to add a new container because there is no running container.
   And at this point, it will try to reuse warmed(paused) containers first.
   If there are warmed containers in invokers, they are chosen first.
   Since the data plane and control plane are separated in FPCScheduler, frequent pausing can aggravate the performance.
   It's same in the old scheduler too but in FPCScheduler there is one more step for containers to send FetchRequest so latency could be a little bit more harmed.
   
   And I feel it's a kind of tradeoff for the VM environment. (In K8S, no container is paused.)
   No matter which value we choose, if the incoming interval for activations is a little bigger than that, the action will suffer from this. (This paper addressed this well: [Serverless in the wild](https://www.usenix.org/conference/atc20/presentation/shahrad), maybe we can consider taking a similar approach)
   
   Anyway, one thing we can see is a bigger value will prevent frequent actions to be suffered.
   Also, let's say unpausing takes around 200~500ms, if the interval is 1s, it's 20~50% overhead of the interval.
   But if the interval is 1 min or 1 hour, 200~500ms overhead might be acceptable.
   
   


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